The present invention relates to an apparatus for measuring a vector voltage ratio, and more particularly, to an apparatus for measuring an amplitude ratio and a phasic difference of AC signals and determining the impedance of a sample element by obtaining a vector voltage ratio of two AC signals.
FIG. 1 is a circuit diagram showing a circuit for measuring impedance Z of a sample element 1. This circuit is arranged such that an input voltage V is impressed on the sample element 1, and an output voltage I is obtained from an operational amplifier 3 with a feedback resistor R. A vector voltage ratio is given by I/V=R.multidot.(1/Z), so that the impedance Z can be measured. V and I can, as shown in FIG. 2, be expressed with respect to a reference voltage E. Since the equation, I/V=x+jy=(I.sub.0 +jI.sub.90)/(V.sub.0 +jV.sub.90), is established, x, y, and z can be obtained by measuring I.sub.0, I.sub.90, V.sub.0, V.sub.90 and performing necessary computation. Such a principle of measurement is described in the specification of U.S. Pat. No. 4,196,475.
There have heretofore been known two types of apparatuses for making such measurement, i.e., seeking V.sub.0, V.sub.90, I.sub.0, I.sub.90. FIG. 3 is a block diagram of the apparatus disclosed in the aforementioned specification of the above identified U.S. Patent. FIG. 4 is a diagram showing the order of measurement and the specified components, which are to be provided, of the two AC signals. The input voltage V and the output voltage I are applied via a switch 5 to one input terminal of a phase detector 7. To the other input terminal of the phase detector 7 are applied signals having a selected phase (0.degree. or 90.degree.) from a variable phase signal generator 6. Output signals of the phase detector 7 are digitized by an A/D converter (ADC) 8 (for instance, a dual slope type ADC) and are then outputted. As illustrated in FIG. 4, the switch is sequentially changed over to the contacts a and b, and at the same instant the phase of the variable phase signal generator 6 is selected, whereby V.sub.0, V.sub.90, I.sub.0 and I.sub.90 are consecutively obtained. That is, a serial type process is effected.
This apparatus employs the phase detector 7 and the ADC 8 in common with respect to the measurements of the above-described four values and then seeks the vector voltage ratio thereof. Consequently, the conversion factor (transfer characteristics) of the phase detector 7 or the like is cancelled, thereby making a highly accurate measurement possible. However, the measurements of the four values are consecutively and serially performed, which unfavorably results in a decrease in the rate of processing.
FIG. 5 is a block diagram of another conventional measuring apparatus. FIG. 6 is a diagram showing the specified components, which are to be provided, of the two AC signals. The difference between the apparatuses of FIGS. 3 and 5 is that in FIG. 5 the phase detectors 9 to 12 and ADCs 13 to 16 are separately utilized for measuring V.sub.0, V.sub.90, I.sub.0, I.sub.90 Therefore, V.sub.0, V.sub.90, I.sub.0, I.sub.90 are concurrently provided by the ADCs 13 to 16, thus carrying out a parallel type process.
Therefore, the apparatus of FIG. 5 is capable of performing a high speed measurement because of simultaneously measuring the four values. However, there are errors, such as errors caused by temperature changes in transfer characteristics of the circuit for measuring the four values causing errors in gain and indicated phase, thus contributing to errors in the accuracy of measurement.
Accordingly, it is a primary object of the present invention to obviate the above-described defects by providing an apparatus capable of selectively executing a serial type process and a parallel type process.
It is another object of the present invention to provide an apparatus which has advantages both of the serial type process and of the parallel type process.
It is another object of the present invention to provide an apparatus capable of performing a more accurate measurement than the conventional serial type process.